1. Field of the Invention
The present invention relates to a variable attenuator, and in particular to a variable attenuator capable of maintaining input/output impedance matching in an attenuation mode.
2. Description of the Related Art
In a mobile communication system, a base station includes a receiver. The receiver adjusts reception sensitivity of reception paths by adjusting a signal to noise ratio (S/N) by attenuating a radio frequency signal transmitted from a mobile communication terminal with a variable attenuator.
In general, an attenuation value of the variable attenuator is controlled by a control voltage. The variable attenuator is required to have attenuation characteristics in linear proportions to a control voltage and have small insertion loss when it does not attenuate a reception signal.
Accordingly, in order to maintain the attenuation characteristics linearly, the variable attenuator has to be designed so as to maintain impedance matching of an input/output block although an attenuation adjustment range (high attenuation mode) is increased in a high frequency circuit.
FIG. 1 is a circuit diagram illustrating a conventional variable attenuator. As depicted in FIG. 1, the conventional variable attenuator includes capacitors C1 and C2 serial-connected between an input terminal RFin and an output terminal RFout; resistors R1 and R2 parallel-connected to the input and output terminals Rfin and RFout respectively; an inductor L1 parallel-connected between the capacitors C1 and C2; an inductor L2 for receiving a control voltage; a PIN diode D1 serial-connected between the inductors L1 and L2; and a capacitor C3 parallel-connected to an anode of the PIN diode D1. The capacitors C1 and C2 remove DC element from a RF signal, and the inductors L1 and L2 remove RF element from a control voltage.
In general, resistance element of the PIN diode D1 is varied according to a DC bias (control voltage). It is possible to adjust attenuation of the PIN diode D1. In order to increase attenuation of the RF signal, a DC bias is increased. In order to decrease attenuation of the RF signal, a DC bias is decreased. In addition, in order not to attenuate the RF signal, a DC bias is not applied.
When a DC bias of 0V (e.g., a control voltage of 0V) is applied, the variable attenuator is operated in a low attenuation mode. In the low attenuation mode, the PIN diode D1 is turned off by the control voltage of 0V, and accordingly internal resistance approaches infinity (∞).
The RF signal applied through the input terminal RFin is transmitted to the output terminal RFout through the capacitors C1 and C2 without being attenuated. Input/output impedance is maintained as 50Ω by the resistors R1 and R2.
By increasing the control voltage, the PIN diode D1 reaches a turn-on voltage (0.6˜0.7V), the PIN diode D1 is turned on, and resistance of the PIN diode D1 is decreased. Because the RF signal transmitted through the capacitor C1 leaks to the ground through the PIN diode D1 and the capacitor C3, the RF signal is rapidly attenuated. The capacitor C3 sets an attenuated frequency range.
When the control voltage is not less than 1.3V, the variable attenuator is operated in a high attenuation mode, because resistance of the PIN diode D1 is 0Ω in the high attenuation mode. The RF signal transmitted through the capacitor C1 leaks to the ground through the PIN diode D1 and the capacitor C3. Accordingly, any RF signal is not transmitted to the output terminal RFout. Herein, input/output impedance is 0Ω.
In a conventional attenuator, in the low attenuation mode having the control voltage of 0V, input/output impedance is maintained at 50Ω. In the high attenuation mode having the control voltage not less than 1.3V, input/output impedance is 0Ω. In general, the attenuator has to maintain impedance matching regardless of attenuation of the PIN diode. That is, it has to maintain input/output impedance at 50Ω continually in the low attenuation mode and the high attenuation mode in order to perform stable matching with other elements.
In a conventional variable attenuator, impedance matching can be maintained in the low attenuation mode. However, impedance matching cannot be maintained in the high attenuation mode, and accordingly it is impossible to secure stability of a system. Accordingly, when the conventional attenuator is used for a LNA (low noise amplifier) of a reception block, pass band characteristics of a duplexer and a high frequency filter have to be varied. In particular, the reception level of a RF signal is lowered due to impedance mismatching with an antenna. As such, a method and system is needed to overcome the problems associated with the related art attenuator.